Atmospheric Sciences & Global Change Research Highlights

New Imaging Tool Directly Measures Liquid Surfaces

The schematic of a vacuum compatible aqueous SIMS device. Insert shows the secondary H- and Si- ion images of this aperture. Enlarge Image

Results: A unique
chemical imaging tool readily and reliably presents volatile liquids to scientific
instruments, according to a team including Pacific Northwest National
Laboratory. These instruments require samples be held in a vacuum, which is
often incompatible with hydrocarbons and other liquids. Designed and built at PNNL,
this one-of-a-kind sample holder continuously pumps the liquid through a
gold-coated microfluidic chamber. The extremely narrow channel provides high
linear velocity at the detection window and helps overcome the liquids'
tendency to vaporize. Instruments access the liquid via an open viewing port.
Tests with electron microscopes and mass spectrometers prove the device can
operate continuously for up to 8 hours. Further, the device handles complex
liquids.

"Great discoveries often require great tools,"
said Dr. Louis Terminello, who leads chemical imaging work at PNNL. "The
discoveries needed to solve today's problems aren't something that you're going
to get by eyeballing a sample and analyzing a spreadsheet."

Why It Matters: Quick,
affordable methods of storing electricity from wind turbines. Effectively
trapping and removing industrial solvents from soil. These and other endeavors need
precise knowledge of interfacial reactions, the interactions that occur between
a liquid and solid or two liquids. This study demonstrated that the
PNNL-created device allows surface tools such as microscopes and secondary ion mass
spectrometers to obtain the needed data on samples and providing answers
scientists need.

"Our device opens a window to
observe interactions between liquid/solids and liquid surfaces, which are relevant
to liquid/solid heterogeneous catalysis and energy storage techniques,"
said Dr. Xiao-Ying Yu at PNNL, who worked on the study.

Methods: Scanning
electron microscopy, time-of-flight secondary ion mass spectrometry, and other surface
characterization techniques often require the sample to be in a vacuum.
Starting in 2010, the team used soft lithography to
fabricate the device's microchannel. The channel was coated in gold to reduce
the liquid's permeation into the device. A pump forces liquid through the
channel continuously. Above the channel is a window open to the instrument's
vacuum, allowing the instrument's electron or other beams access to the sample.

"Using our microfluidic
device is advantageous compared to certain other techniques as it allows us to
observe these complicated systems in their native environments," said Dr.
Theva Thevuthasan, who led the research and is part of PNNL's Chemical Imaging
Initiative.

The team examined the surface of liquids
of increasing complexity. Many of these liquids were provided by SPI
Supplies/Structure Probe, Inc. These liquids included
organic solvents in aqueous solutions and IgG-conjugated gold nanoparticles in
an aqueous solvent. The team used the device with energy-dispersive X-ray
techniques in EMSL's scanning electron microscope and in the time-of-flight
secondary ion mass spectrometer. The experiments proved the device could
provide better detection of nanoparticles suspended in the liquid and
identifications of characteristic fragments of IgG in the flow cell compared
with conventional dry samples or wet samples in the ESEM mode. Moreover, it can be safely run continuously
for 8 hours and could handle high fluence beams for up to 30 minutes.

The microfluidic device is
available for use on instruments at EMSL. Researchers from around the world can
use EMSL instrumentation free of charge via the DOE user facility's proposal
system.

What's Next? The
researchers are reaching out to scientists at universities to get more
scientists using the microfluidic sample chamber, which can be applied already
in some systems without more development.